1. Field of the Invention
The present invention relates to the adaptation of an electronic system, called a source, transmitting an analog video signal, generally a variable voltage, to another electronic circuit, called a receiver, via transmit means.
The present invention relates to the transmission of a video signal from a source which for example corresponds to a reader of a DVD-type video disk (Digital Versatile Disk), to a camera or to a decoder box (Set Top Box) and a receiver, which for example corresponds to a display screen or to a video recording system. The transmit means connecting the source to the receiver may correspond to a cable. For the video signal received by the receiver to be as little deformed as possible, it is necessary for the source impedance and the receiver impedance to be equal to the characteristic impedance of the transmit means. It can then be said that the connection is adapted. The characteristic impedance of a cable most used for the transmission of a video signal is 75 ohms
2. Discussion of the Related Art
Different international standards, for example, standard EIA, define the features of the video signals used for such transmissions. Currently, to perform a transmission with the best possible quality while respecting the existing standards, the video signal comprises a non-zero D.C. component which is transmitted to the receiver. Such a connection is designated as DC and a receiver capable of receiving a video signal with a non-zero D.C. component is called a DC receiver.
FIG. 1 schematically shows a source 10 of a video signal SOUT connected to a DC receiver 12 by a cable 14. Source 10 comprises an output stage 16 comprising a generator 18 receiving a video signal SVIDEO and providing a video signal SOUT. Generator 18 is connected to a source of a reference voltage 19, generally the ground of source 10. A resistor 20 is provided between the output of generator 18 and an output terminal O of source 10. Cable 14 is connected between terminal O and an input terminal I of receiver 12. DC receiver 12 comprises a resistor 22 connected between terminal I and a source of a reference voltage 24, generally the ground of receiver 12. To obtain an adapted connection, resistors 20 and 22 have the same value as the characteristic impedance of cable 14.
There exist certain standards, for example, Japanese standards, which require that the video signal transmitted over the cable to comprise no D.C. component and which, for this purpose, provide for the receiver to comprise a capacitive element in series with a resistive element to eliminate the D.C. component of the video signal provided by the source. Such a connection is known as an AC connection and a receiver capable of receiving a video signal with a zero D.C. component is called an AC receiver.
FIG. 2 shows a diagram similar to FIG. 1 in the case of an A.C. connection. AC receiver 12 comprises a capacitor 26 series-assembled between terminal I and resistor 22.
The receiver to which the video signal source can be connected has an input impedance which may thus be purely resistive or comprise a resistive component and a capacitive component. In the case of a DC receiver, the source must be able to supply current while in the case of an AC receiver, the source must be able to both supply and absorb current.
FIG. 3 shows a conventional example of embodiment of a video signal source capable of being connected to a DC receiver or to an AC receiver. Output stage 16 comprises a circuit of emitter follower type comprising a differential amplifier 25 having its positive terminal (+) receiving video signal SVIDEO and having its negative terminal (−) connected to a node E. A resistor Rg1 is provided between node E and a source of a reference voltage VREF. A resistor Rg2 is provided between node E and a node F. The output of amplifier 25 drives the base of an NPN-type bipolar transistor Tbuf having its collector connected via a resistor Rbuf to a source of a reference voltage 27, for example, the positive supply of source 10, and having its emitter connected to node F. Resistor 20 is arranged between nodes F and O. A current generator 28 is arranged between node F and ground 19. Source 10 is likely to absorb and supply current and can thus be connected to an AC receiver or to a DC receiver. However, such a source 10 has the disadvantage of a strong consumption since current generator 28 supplies current uselessly when it is connected to a DC receiver.